Routing devices within a network, often referred to as routers, maintain routing information that describe available routes through the network. Upon receiving an incoming packet, the routers examine information within the packet and forward the packet in accordance with the routing information. In order to maintain an accurate representation of the network, routers exchange routing information in accordance with one or more defined routing protocols, such as the Border Gateway Protocol (BGP).
Multi-protocol Label Switching (MPLS) is a mechanism used to engineer traffic patterns within Internet Protocol (IP) networks. By using MPLS, a source device can request a path through a network, i.e., a Label Switched Path (LSP). An LSP defines a distinct path through the network to carry MPLS packets from the source device to a destination device. A short label associated with a particular LSP is affixed to packets that travel through the network via the LSP. Routers along the path cooperatively perform MPLS operations to forward the MPLS packets along the established path. LSPs may be used for a variety of traffic engineering purposes including bandwidth management and quality of service (QoS). A packet may be a formatted set of data.
A variety of protocols exist for establishing LSPs. For example, one such protocol is the label distribution protocol (LDP). Another type of protocol is a resource reservation protocol, such as the Resource Reservation Protocol with Traffic Engineering extensions (RSVP-TE). RSVP-TE uses constraint information, such as bandwidth availability, to compute paths and establish LSPs along the paths within a network. RSVP-TE may use bandwidth availability information accumulated by a link-state interior routing protocol, such as the Intermediate System—Intermediate System (ISIS) protocol or the Open Shortest Path First (OSPF) protocol.
Head-end routers of an LSP are commonly known as ingress routers, while routers at the tail-end of the LSP are commonly known as egress routers. Ingress and egress routers, as well as intermediate routers along the LSP that support MPLS, are referred to generically as label switching routers (LSRs). A set of packets to be forwarded along the LSP is referred to as a forwarding equivalence class (FEC). A plurality of FECs may exist for each LSP, but there may be only one active LSP for any given FEC. Typically, a FEC definition includes the IP address of the destination of the LSP, e.g., an IP address assigned to the egress router of the LSP. The ingress label edge router (LER) uses routing information, propagated from the egress LER, to determine the LSP, to assign labels for the LSP, and to affix a label to each packet of the FEC. The LSRs use MPLS protocols to receive MPLS label mappings from downstream LSRs and to advertise MPLS label mappings to upstream LSRs. When an LSR receives an MPLS packet from an upstream router, it switches the MPLS label according to the information in its forwarding table and forwards the packet to the appropriate downstream LSR or LER. The egress LER removes the label from the packet and forwards the packet to its destination in accordance with non-label based packet forwarding techniques.
In general, each router along the LSP maintains a context that associates a FEC with an incoming label and an outgoing label. In this manner, when an LSR receives a labeled packet, the LSR may swap the label (i.e., the incoming label) with the outgoing label by performing a lookup in the context. The LSR may then forward the packet to the next LSR or LER along the LSP. The next router along the LSP is commonly referred to as a downstream router or a next hop.
In some instances, a node or link along an LSP may no longer be available. For example, a link along the LSP, or a node may experience a failure event, such as when one or more components of a router fail or the router is brought down by a user, such as a network operator. In these instances, signaling of a new LSP would fail when the LSP was to be explicitly routed along a path that traverses the unavailable link or node. An LSR along the path of the new LSP would detect the failed link or node, and may send an error message indicating that the new LSP cannot be established as requested.
The connection between two devices on a network is generally referred to as a link. Connections between devices of different autonomous systems are referred to as external links while connections between devices within the same autonomous system are referred to as internal links. Many conventional computer networks, including the Internet, are designed to dynamically reroute data packets in the event an individual link fails. Upon failure of a link, the routers transmit new connectivity information to neighboring devices, allowing each device to update its local routing table. Links can fail for any number of reasons, such as failure of the physical infrastructure between the devices, or failure of the devices interfacing with the link.
When a link or router in the network fails, routers using traditional link state protocols, such as OSPF and/or IS-IS, may take a long time to adapt their forwarding tables in response to the topological change resulting from node and/or link failures in the network. The process of adapting the forwarding tables is known as convergence. This time delay occurs because each node must update its representation of the network topology and execute the shortest path algorithm to calculate the next-hop for each destination within the updated network topology. Until the next-hops are re-computed, traffic being sent toward the failed links may be dropped. Some deployments take as much as 500 milliseconds to several seconds for detection and recovery from failures in the network. These large convergence times may adversely affect the performance of applications such as Voice over Internet Protocol (VoIP) and multimedia applications, which often rely on RSVP-TE, and are extremely sensitive to traffic loss. A resource reservation protocol, such as RSVP-TE, may be used to traffic-engineer paths through a network for sending network traffic associated with these types of traffic-loss-sensitive applications.